The minimum feature sizes of integrated circuits (ICs) have been shrinking for years. Commensurate with this size reduction, various process limitations have made IC fabrication more difficult. One area of fabrication technology in which such limitations have appeared is photolithography.
An integral component of photolithographic apparatus is a “reticle” which includes a pattern corresponding to features at one layer in an IC design. As light passes through the reticle, it is refracted and scattered by the chromium edges. This causes the projected image to exhibit some rounding and other optical distortion. While such effects pose relatively little difficulty in layouts with large feature sizes (e.g., layouts with critical dimensions above about 1 micron), they cannot be ignored in layouts having features smaller than about 1 micron. The problems become especially pronounced in IC designs having feature sizes near the wavelength of light used in the photolithographic process.
To remedy this problem, a reticle correction technique known as optical proximity correction (OPC) has been developed. Optical proximity correction involves adding dark regions to and/or subtracting dark regions from a reticle design at locations chosen to overcome the distorting effects of diffraction and scattering. Typically, OPC is performed on a digital representation of a desired IC pattern. First, the digital pattern is evaluated with software to identify regions where optical distortion will result, and a digital representation of a reticle or photomask design is modified to create an optically corrected or OPC mask. The modification is performed by a computer having appropriate software for performing OPC. A mask simulator is then used to emulate the wafer printing characteristics of the OPC mask during optical lithography, resulting in an OPC aerial image. The mask may be corrected by moving portions of the edges as necessary to improve the simulated wafer results.
Before correction of the mask can take place, the boundaries of the mask design must be fragmented into edge segments. The fragmented portions of the boundaries, or edge segments, are then moved to obtain a wafer print image which is the most close to the target design. Usually, the edge segments are moved in a direction normal to the boundary edge, which create fragmentation regions along the mask boundaries. After the moves are calculated, the moves are “applied” to the mask design to obtain the new OPC features. One problem with conventional mask correction techniques is that it makes the OPC mask very complex at locations where fragmentation regions meet at an angle, as shown in FIGS. 1A and 1B.
FIG. 1A is a diagram illustrating a portion of an original mask feature 10 having boundary edges that have been moved to create fragmentation regions 12 and 14. The location where the two fragmentation regions 12 and 14 meet create a jog 16 in the mask geometry that may be too complex for the mask making process to pattern. Therefore, there have been attempts to reduce the number of jogs 16 that are present in the final OPC mask by averaging or smoothing the neighboring fragmentation regions. This is accomplished by connecting the ends of the fragmentation regions 12 and 14 together, as shown in FIG. 1B.
One problem with the above smoothing correction technique is that the fragmentation process may result in a move that violates design rules. For example, a high-density chip design may require a distance of no less than 0.8λ exist among feature boundaries. It is possible, however, that the edges of the corrected mask are moved in a manner that violates this design rule constraint. Another problem is that most mask design processes require that the boundaries of the mask be in a horizontal, vertical, or 45° orientation. The process for reducing the number of jogs near angles by connecting the ends of fragmentation regions may result in a non-45° geometry, as shown in FIG. 1B, which cannot be used.
Accordingly, what is needed is an improved method and system performing a OPC edge correction based on a smoothed mask design. The present invention addresses such a need.